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International audience; The Paleocene lavas from Dianzhong Formation (E1d) in Linzhou basin of southern Lhasa terrane are a key target for paleomagnetic investigations into the timing and paleolatitude of the initial India-Asia collision. Controversy exists, however, on whether these rocks preserve a primary remanent magnetization. Here we reanalyze previously published thermal demagnetization data and report detailed rock magnetic results and petrographic observations of these rocks. We find that the original magnetic carrier, a magmatic multidomain Ti-poor titanomagnetite, underwent significant grain size reduction and was variably reacted to single-domain maghemite and nano-hematite. Such strong alteration may have resulted from successive hydrothermal events: a first event related to the ∼52 Ma dike intrusions into the E1d that accompanied a massive ignimbrite eruption deposited above the E1d producing heating up to 300 °C; and a secondary event related to the 42 - 27 Ma southward overthrusting of the basin, heating the E1d up to 130 – 145 °C. Unblocking/inversion temperature spectra of the authigenic maghemite and nano-hematite overlap with those of the titanomagnetite, implying that the primary remanence of the E1d lavas has been contaminated or replaced by thermoviscous and chemical remanent magnetizations. Thus the isolated characteristic remanent magnetization from these rocks, whether slightly or completely altered, cannot be considered primary and should not be used for paleolatitudinal determination. Our study confirms that hydrothermal alteration can seriously jeopardize the remanence carried by titanomagnetite and thus should be tested for paleomagnetic investigations of rock units from tectonically active areas.

The drift history of the Lhasa terrane from Gondwana to Asia plays a crucial role in understanding the Tethys evolution and true polar wander (TPW). However, few reliable paleomagnetic results from Jurassic strata are currently available for reconstructing its northward journey. We performed a combined paleomagnetic and geochronological study on Bima Formation strata in the Xigaze area. Combined with previous results from the Sangri area, our results reveal a paleolatitude of 8 ± 4°S at ∼180 Ma for the reference point (29.3°N, 90.3°E). Along with other paleomagnetic results from the Triassic to Cretaceous, our new results suggest that the Lhasa terrane motion accelerated from ∼2 cm/yr during ∼220–180 Ma to ∼17 cm/yr during ∼180–170 Ma. Paleolatitude information of the North Qiangtang terrane and Tethyan Himalaya is calculated from paleopoles that meet five criteria, which include (a) structural control, (b) well-determined rock age, (c) stepwise demagnetizations, (d) a minimum of 25 specimens or 8 sites are contained, and (e) robust field or reversal tests are provided. Both terranes also show significant acceleration during their northward motion, which may be related to oceanic slab subduction. Thus, all Gondwana-derived microcontinents seem to share a significant acceleration during their northward motion. In addition, recent paleomagnetic results from volcanic rocks dated at ∼155 Ma subdivide the overall northward motion during ∼170–130 Ma into two stages, which include a southward drift during ∼170–155 Ma followed by northward motion during ∼155–130 Ma. These results support the fast Late Jurassic TPW during a ∼10 Myr time span.

International audience; Editors of JGR-Solid Earth express their appreciation to those who served as peer reviewers for the journal in 2021.

This study was financially supported by the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (grants QYZDB-SSW-DQC021, QYZDY-SSW-DQC001, and ZDBS-SSW-DQC001).

International audience; The editors of JGR: Solid Earth announce that Plain Language Summaries will be required for all manuscripts.

The Miocene epicontinental Paratethys Sea of central Eurasia has experienced multiple restriction and reconnection events to the open ocean. Magnetostratigraphy is an important dating tool to better understand the temporal and spatial paleoenvironmental variations associated with these changes. Magnetostratigraphy in the Paratethys domain, however, is complicated by the presence of greigite (Fe3S4). Here we report rock magnetic and X-ray fluorescence data of the Tisa section (Romania) which was previously magnetostratigraphically dated at the middle Miocene (base at 12.8 Ma and top at 12.2 Ma). This section comprises the Badenian Sarmatian Extinction Event (BSEE), which is marked by a major salinity change from marine to brackish environments, related to the opening of the connection between the Central and the Eastern Paratethys basins. In the marine Badenian sediments below the BSEE, the pyritization process is shown to be complete because of abundant sulfate supply. In the brackish Sarmatian deposits, four intervals with early diagenetic greigite are observed, and linked to insufficient sulfate in the water column. These four greigite intervals appear to correspond to maxima in the ∼100 kyr eccentricity cycle. We propose that increased fresh water from the Eastern Paratethys basin during eccentricity maxima restricted the sulfate availability in the Tisa area, leading to a reduced HS− production and enhanced greigite preservation. The early diagenetic formation of greigite enables a quasi syn-depositional recording of the paleomagnetic field, which allows reliable paleomagnetic dating in this section. Our results further suggest greigite as a potential indicator for salinity changes during marine/brackish transitions.

In their comment on our publications on pervasive remagnetization of Jurassic-Paleogene carbonate rocks of the Tibetan Himalaya [Huang et al., 2017, Journal of Geophysical Research: Solid Earth, 122, doi: 810.1002/2016JB013662 and 122, doi: 10.1002/2017JB013987], Yi et al. [this volume] questioned our fold tests applied to their published paleomagenetic results from the Paleogene Zongpu and latest Cretaceous Zongshan carbonate rocks [Patzelt et al., 1996, Tectonophysics, 259(4), 259-284; Yi et al., 2011, Earth and Planetary Science Letters, 309(1), 153-165]. They argued that authigenic magnetite pseudomorphic after pyrite, which is the dominant magnetic carrier within these carbonate rocks as indicated by our thorough rock magnetic and petrographic investigations, was formed during early diagenesis and that the primary natural remanent magnetization (NRM) is retained by these carbonate rocks. However, their statement for the invalidity of our fold tests is based on unrealistic assumptions that these carbonate rocks carry primary NRM and that the upper Zongpu Formation was deposited on a 10° primary dip. Their argument for immediate oxidization of pyrite to authigenic magnetite after carbonate deposition onto the continental passive margin ignores that sulphate-reducing conditions were prevailing during early diagenesis, it is also inconsistent with the timing of the secondary remanence acquisition in remagnetized carbonate rocks elsewhere. As previously demonstrated, and agreed upon by Yi et al. [this volume], the Zongpu and Zongshan carbonate rocks in Gamba are remagnetized; here we argue that the timing of remagnetization cannot be demonstrated to shortly post-date sedimentation. These data should therefore not be used for tectonic reconstructions.

Netherlands Organization for Scientific Research (NWO) with a Rubicon grant [825.15.016]; Institute for Rock Magnetism (IRM) at the University of Minnesota; Instruments and Facilities program of NSF; ERC Starting Grant [306810]; NWO VIDI [864.11.004]

Netherlands Organization for Scientific Research (NWO) [825.15.016]; Institute for Rock Magnetism (IRM) at the University of Minnesota - Instruments and Facilities Program of NSF

The Early Devonian geological time scale (base of the Devonian at 418.8 ± 2.9 Myr , Becker et al., 2012 ) suffers from poor age control, with associated large uncertainties between 2.5 and 4.2 Myr on the stage boundaries. Identifying orbital cycles from sedimentary successions can serve as a very powerful chronometer to test and, where appropriate, improve age models. Here, we focus on the Lochkovian and Pragian, the two lowermost Devonian stages. High-resolution magnetic susceptibility ( χ in – 5 to 10 cm sampling interval) and gamma ray spectrometry (GRS – 25 to 50 cm sampling interval) records were gathered from two main limestone sections, Požar-CS (118 m, spanning the Lochkov and Praha Formations) and Pod Barrandovem (174 m; Praha Formation), both in the Czech Republic. An additional section (Branžovy, 65 m, Praha Formation) was sampled for GRS (every 50 cm). The χ in and GRS records are very similar, so χ in variations are driven by variations in the samples' paramagnetic clay mineral content, reflecting changes in detrital input. Therefore, climatic variations are very likely captured in our records. Multiple spectral analysis and statistical techniques such as: Continuous Wavelet Transform, Evolutive Harmonic Analysis, Multi-taper method and Average Spectral Misfit, were used in concert to reach an optimal astronomical interpretation. The Požar-CS section shows distinctly varying sediment accumulation rates. The Lochkovian (essentially equivalent to the Lochkov Formation (Fm.)) is interpreted to include a total of nineteen 405 kyr eccentricity cycles, constraining its duration to 7.7 ± 2.8 Myr . The Praha Fm. includes fourteen 405 kyr eccentricity cycles in the three sampled sections, while the Pragian Stage only includes about four 405 kyr eccentricity cycles, thus exhibiting durations of 5.7 ± 0.6 Myr and 1.7 ± 0.7 Myr respectively. Because the Lochkov Fm. contains an interval with very low sediment accumulation rate and because the Praha Fm. was cross-validated in three different sections, the uncertainty in the duration of the Lochkov Fm. and the Lochkovian is larger than that of the Praha Fm. and Pragian. The new floating time scales for the Lochkovian and Pragian stages have an unprecedented precision, with reduction in the uncertainty by a factor of 1.7 for the Lochkovian and of ∼6 for the Pragian. Furthermore, longer orbital modulation cycles are also identified with periodicities of ∼1000 kyr and 2000–2500 kyr.

Carbonate rocks, widely used for paleomagnetically quantifying the drift history of the Gondwana derived continental blocks of the Tibetan Plateau and evolution of the Paleo/Meso/Neo-Tethys Oceans, are prone to pervasive remagnetization. Identifying remagnetization is difficult because it is commonly undetectable through the classic paleomagnetic field tests. Here we apply comprehensive paleomagnetic, rock magnetic, and petrographic studies to upper Triassic limestones in the eastern Qiangtang block. Our results reveal that detrital/biogenic magnetite, which may carry the primary natural remanent magnetization (NRM), is rarely preserved in these rocks. In contrast, authigenic magnetite and hematite pseudomorphs after pyrite, and monoclinic pyrrhotite record three episodes of remagnetization. The earliest remagnetization was induced by oxidation of early diagenetic pyrite to magnetite, probably related to the collision between the northeastern Tibetan Plateau and the Qiangtang block after closure of the Paleo-Tethys Ocean in the Late Triassic. The second remagnetization, residing in hematite and minor goethite, which is the further subsurface oxidation product of pyrite/magnetite, is possibly related to the development of the localized Cenozoic basins soon after India-Asia collision in the Paleocene. The youngest remagnetization is a combination of thermoviscous and chemical remanent magnetization carried by authigenic magnetite and pyrrhotite, respectively. Our analyses suggest that a high supply of organic carbon during carbonate deposition, prevailing sulfate reducing conditions during early diagenesis, and widespread orogenic fluid migration related to crustal shortening during later diagenesis, have altered the primary remanence of the shallow-water Tethyan carbonate rocks of the Tibetan Plateau. We emphasize that all paleomagnetic results from these rocks must be carefully examined for remagnetization before being used for paleogeographic reconstructions. Future paleomagnetic investigations of the carbonate rocks in orogenic belts should be accompanied by thorough rock magnetic and petrographic studies to determine the origin of the NRM. (C) 2019 Elsevier B.V. All rights reserved.

Members of the editorial board of JGR-Solid Earth express their appreciation to those who served as peer reviewers for the journal in 2020.

Today's paleomagnetic and magnetic proxy studies involve processing of large sample collections while simultaneously demanding high quality data and high reproducibility. Here we describe a fully automated interface based on a commercial horizontal pass-through “2G” DC-SQUID magnetometer. This system is operational at the universities of Bremen (Germany) and Utrecht (Netherlands) since 1998 and 2006, respectively, while a system is currently being built at NGU Trondheim (Norway). The magnetometers are equipped with “in-line” alternating field (AF) demagnetization, a direct-current bias field coil along the coaxial AF demagnetization coil for the acquisition of anhysteretic remanent magnetization (ARM) and a long pulse-field coil for the acquisition of isothermal remanent magnetization (IRM). Samples are contained in dedicated low magnetization perspex holders that are manipulated by a pneumatic pick-and-place-unit. Upon desire samples can be measured in several positions considerably enhancing data quality in particular for magnetically weak samples. In the Bremen system, the peak of the IRM pulse fields is actively measured which reduces the discrepancy between the set field and the field that is actually applied. Techniques for quantifying and removing gyroremanent overprints and for measuring the viscosity of IRM further extend the range of applications of the system. Typically c. 300 paleomagnetic samples can be AF demagnetized per week (15 levels) in the three-position protocol. The versatility of the system is illustrated by several examples of paleomagnetic and rock magnetic data processing.

Fluid infiltration within fault zones is an important process in earthquake rupture. Magnetic properties of fault rocks convey essential clues pertaining to physicochemical processes in fault zones. In 2011, two shallow holes (134 and 54 m depth, respectively) were drilled into the Yingxiu-Beichuan fault (Longmen Shan thrust belt, China), which accommodated most of the displacement of the 2008 Mw 7.9 Wenchuan earthquake. Fifty-eight drill core samples, including granitic host rock and various fault rocks, were analyzed rock-magnetically, mineralogically, and geochemically. The magnetic behavior of fault rocks appears to be dominated by paramagnetic clay minerals. Magnetite in trace amounts is identified as the predominant ferrimagnetic fraction in all samples, decreasing from the host rock, via fault breccia to (proto-)cataclasite. Significant mass-losses (10.7–45.6%) are determined for the latter two with the “isocon” method. Volatile contents and alteration products (i.e., chlorite) are enriched toward the fault core relative to the host rocks. These observations suggest that magnetite depletion occurred in these fault rocks—exhumed from the shallow crust—plumbed by fluid-assisted processes. Chlorite, interpreted to result from hydrothermal activity, occurs throughout almost the entire fault core and shows high coefficients of determination (R2 > 0.6) with both low and high-field magnetic susceptibility. Close relationships, with R2 > 0.70, are also observed between both low and high-field magnetic susceptibility and the immobile elements (e.g., TiO2, P2O5, MnO), H2O+, and the calculated mass-losses of fault rocks. Hence, magnetic properties of fault rocks can serve as proxy indicators of fluid infiltration within shallow fault zones.

Magnetic property research in the geosciences has seen an enormous development during the most recent decade. The underlying special collection published in two AGU journals: Journal of Geophysical Research—Solid Earth and Geochemistry, Geophysics, Geosystems is an outcome of the 11th International Conference on Rock Magnetism, held in Utrecht (The Netherlands) in July 2017 and reflects this full breadth. We recognize four groups in which individual contributions are gathered. (1) Recent advances in fundamental rock and mineral magnetic methods, including discussions on how to assess magnetic grain size, which is centrally important to magnetic stability over geologic timescales and high-fidelity paleomagnetic recording. (2) Geomagnetism with an emphasis on late Pleistocene paleosecular variation and relative paleointensity records, relevant to geomagnetic field models. (3) Reassessments of classic paleomagnetic field tests and examples of tectonic and magnetostratigraphic applications. (4) The merit of magnetic property analysis in solving geoscience problems, both in soft and hard rocks.

Paleomagnetic dating of the India-Asia collision hinges on determining the Paleogene latitude of the Lhasa terrane (southern Tibet). Reported latitudes range from 5°N to 30°N, however, leading to contrasting paleogeographic interpretations. Here we report new data from the Eocene Linzizong volcanic rocks in the Nanmulin Basin, which previously yielded data suggesting a low paleolatitude (~10°N). New zircon U-Pb dates indicate an age of ~52 Ma. Negative fold tests, however, demonstrate that the isolated characteristic remanent magnetizations, with notably varying inclinations, are not primary. Rock magnetic analyses, end-member modeling of isothermal remanent magnetization acquisition curves, and petrographic observations are consistent with variable degrees of posttilting remagnetization due to low-temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously. Previously reported paleomagnetic data from the Nanmulin Basin implying low paleolatitude should thus not be used to estimate the time and latitude of the India-Asia collision. We show that the paleomagnetic inclinations vary linearly with the contribution of secondary hematite to saturation isothermal remanent magnetization. We tentatively propose a new method to recover a primary remanence with inclination of 38.1° (35.7°, 40.5°) (95% significance) and a secondary remanence with inclination of 42.9° (41.5°,44.4°) (95% significance). The paleolatitude defined by the modeled primary remanence—21°N (19.8°N, 23.1°N)—is consistent with the regional compilation of published results from pristine volcanic rocks and sedimentary rocks of the upper Linzizong Group corrected for inclination shallowing. The start of the Tibetan Himalaya-Asia collision was situated at ~20°N and took place by ~50 Ma.

Recognition of coring-induced disturbance, which is essential for magnetic fabric and paleomagnetic studies of poorly lithified sediments, is generally not straightforward. Here, we report on anisotropy of magnetic susceptibility (AMS) and paleomagnetic data of the sediments from Holes U1480E and U1480H, IODP Expedition 362, west of the Sumatra subduction zone. AMS is characterized by steep minimum principal axes (Kmin) in undisturbed sediments. However, a considerable portion of the recovered sediments are affected by significant coring-induced disturbance. In these cases, we observed three AMS patterns: (1) AMS principal axes are randomly distributed for sediments with mingling and distortion of beds, (2) Kmin axes of sediments with upward-arching beds are deflected out of the splitting face of the working half, and (3) suck-in sediments are characterized by vertical Kmax axes. These deformation-dependent AMS patterns can be attributed to the realignment of mineral particles caused by the coring process and subsequent sampling procedures. Besides a low-coercivity, vertical, drilling-induced overprint, we observed a high-coercivity component that is likely a composite of the primary magnetization with a demagnetization-resistant portion of the drilling overprint. After accounting for the disturbed intervals, several polarity transitions can be identified in the undisturbed sediments which correlate well with the Pleistocene geomagnetic polarity timescale. These observations demonstrate that great caution is required when attributing geological significance to AMS and paleomagnetic data obtained from soft sediment cores, which are highly susceptible to coring-induced disturbance. In addition, AMS measurements provide a potential tool for identifying core deformation for further paleomagnetic studies.

[1] Pelagic marine carbonates provide important records of past environmental change. We carried out detailed low-temperature magnetic measurements on biogenic magnetite-bearing sediments from the Southern Ocean (Ocean Drilling Program (ODP) Holes 738B, 738C, 689D, and 690C) and on samples containing whole magnetotactic bacteria cells. We document a range of low-temperature magnetic properties, including reversible humped low-temperature cycling (LTC) curves. Different degrees of magnetite oxidation are considered to be responsible for the observed variable shapes of LTC curves. A dipole spring mechanism in magnetosome chains is introduced to explain reversible LTC curves. This dipole spring mechanism is proposed to result from the uniaxial anisotropy that originates from the chain arrangement of biogenic magnetite, similar to published results for uniaxial stable single domain (SD) particles. The dipole spring mechanism reversibly restores the remanence during warming in LTC measurements. This supports a previous idea that remanence of magnetosome chains is completely reversible during LTC experiments. We suggest that this magnetic fingerprint is a diagnostic indicator for intact magnetosome chains, although the presence of isolated uniaxial stable SD particles and magnetically interacting particles can complicate this test. Magnetic measurements through the Eocene section of ODP Hole 738B reveal an interval with distinct magnetic properties that we interpret to originate from less oxidized biogenic magnetite and enrichment of a biogenic “hard” component. Co-occurrence of these two magnetic fingerprints during the late Eocene in the Southern Ocean indicates less oxic conditions, probably due to increased oceanic primary productivity and organic carbon burial.

We present new palaeomagnetic data from Upper Carboniferous and Lower Permian grey and red sediments from the Donets Basin, Ukraine, part of the Palaeozoic East European Platform. Detailed demagnetization of these units reveals two ancient components of magnetization: component "B", which is carried by magnetite and pigmentary haematite, and a high unblocking temperature component "C", present only in the red beds, carried by detrital haematite. The "B" and "C" components both pass fold tests indicating a primary or a near primary-origin for magnetizations. The "C" component, however, yields palaeolatiudes that are consistently lower (by up to ~. 12° of latitude or ~. 1330. km) than those derived from the "B" component, and we argue that this is due to significant inclination-shallowing of the "C"-component. A comparison with European reference palaeomagnetic data reveals that the reference data also span a large spread of palaeolatitudes for this time, and we argue that unrecognized shallowing may have crept into the reference data when based on sedimentary units. A more rigorous approach to selecting reference palaeomagnetic data may well be key to resolving palaeogeographic controversies at this time. © 2010 Elsevier B.V.

Remagnetization of a palaeomagnetic signal is difficult to recognize independently of directional information. The situation becomes more complex when remagnetized rocks pass palaeomagnetic field tests, for example when the remagnetization of a rock sequence has occurred before folding. It is evident that palaeogeographic reconstructions are seriously flawed when actu- ally remagnetized rocks are not identified as such. Here we discuss the merits and pitfalls of so-called end-member modelling of acquisition curves of the isothermal remanent magnetization (IRM) to recognize remagnetized strata. The technique requires no a priori information about the IRM acquisition curves. The algorithm unmixes a set of IRM acquisition curves into a number of invariant curves termed end members and calculates the mixing proportions of the end members for each sample. Since primary natural remanent magnetization (NRM) and remag- netized NRM are acquired by different processes, their signatures can be recognized from subtle differences in the magnetic properties. We illustrate the potential of the approach by three case studies, one from Spain and two from Turkey, in which the magnetic properties of remagnetized and non-remagnetized rocks are evaluated. The natural remanent magnetization (NRM) of a rock is described asremagnetized when itrepresents apalaeomagnetically younger age thanthe sediment age of the rock formation under study. Remagneti- zation interferes with palaeogeographic reconstruc- tions,asisincreasinglybeingappreciated.The'Late Palaeozoic remagnetization event' was firmly rec- ognized in the Appalachians in the USA in the early 1980s (e.g. McCabe et al. 1983; see the review of Van der Voo & Torsvik 2012) while the implica- tionsoflatePalaeozoicremagnetizationinEuropean

SUMMARY In complex sedimentary environments with different sources of the magnetic particles, it is vital that all factors contributing to and affecting the magnetic signals be well understood for a better constrained interpretation of palaeomagnetic and climatic data. Palaeoclimatically driven signals may be convolved with volcanic and cosmogenic sources, and diagenesis may alter the primary magnetic mineral assemblage. Rock magnetic methods and electron microscopy provide suitable means to characterize magnetic assemblages. In this study of core-retrieved sediments from the last 150 kyr off the coast of Senegal (NW Africa), the occurrence of magnetic spherules of volcanic origin and cosmogenic particles appeared to be insignificant. In glacial stages, ferrimagnetic iron and iron-titanium oxides magnetically dominate with few signs of a diagenetic overprint. Low- and room temperature magnetic results reveal evidence for haematite and goethite while haemoilmenite and titanomagnetite were identified from low-temperature AC susceptibility measurements and SEM observations. In contrast, the interglacial samples appeared to have been severely affected by diagenesis, with pyrite making up over 50 per cent of the iron-bearing phases. The Verwey transition observed at 110 K confirms the presence of magnetite. The glacial and interglacial contrast reveals the close link between northwest African climate and the magnetic mineralogy of the sediments.

The Nihewan fluvio-lacustrine sequence (North China) has recorded late Pliocene–Pleistocene climatic and environmental changes, and contains valuable information on early human evolution in high-latitude East Asia. We carried out a combined mineral-magnetic and geochemical investigation on a sequence from the Xiantai section, eastern Nihewan Basin. Results suggest that large-amplitude magnetic-property variations between so-called ‘high-magnetic’ and ‘low-magnetic’ units mainly result from preservation/dissolution cycles of detrital magnetic minerals in alternating oxic and anoxic depositional environments. In our preservation/dissolution model, the high- and low-magnetic units represent glacial and interglacial deposits, respectively. This contributes to a better understanding of the link between magnetic properties and climate in the Xiantai fluvio-lacustrine sequence. Based upon this relationship, early humans may have occupied the Xiantai and Xiaochangliang sites during an interglacial period, during both interglacial and glacial periods at the Donggutuo site, and during a glacial period at the Maliang site. Our work provides a paleoenvironmental context for early human adaptation and occupation in mainland East Asia during the early Pleistocene.

SUMMARY Here we report a detailed mineral magnetic study of Neolithic burnt levels in the Mirador Cave (Sierra de Atapuerca, Burgos, Spain) to reconstruct the burning history and to investigate their suitability for archaeomagnetic purposes. As a consequence of the ancient burning, a characteristic facies sequence was developed along the Holocene stratigraphy. From top to bottom it includes: (i) 2–10 cm ash layers, (ii) ∼2 cm underlying rubefied layers and (iii) clay, mainly unburnt and of variable thickness. In some cases a thermally altered facies (2–6 cm) with a heterogeneous texture was identified (facies TF), usually between rubefactions and the unburnt levels. 126 oriented samples from 4 units (MIR12, 15, 18 and 21) and a 2 m section, all comprised between units MIR21 (6380 ± 40 14C BP) and MIR9 (5090 ± 40 14C BP) were analysed with rock magnetic methods. In addition, bulk sediment from each facies that comprise the Neolithic sequence was investigated. Measurements included: stepwise alternating field and thermal demagnetization of natural remanent magnetization (NRM), viscosity experiments, determination of the anisotropy of the magnetic susceptibility (AMS), the susceptibility frequency dependence at room temperature and determination of the temperature dependence of the susceptibility. Additional experiments consisted in the determination of the behaviour of anhysteretic and isothermal (IRM) remanences, magnetic hysteresis loops, first-order-reversal-curve diagrams, and thermal demagnetization of three-axial IRM. It appeared that the facies all show a fairly similar magnetic mineralogy and grain size dominated by low-titanium magnetite that is often partially maghaemitized. Main differences constitute the amount of superparamagnetic particles that is higher in unburnt strata concurring with a less well-defined NRM behaviour. The magnetic mineral concentration is notably higher in ashes. This homogeneity strongly suggests that similar sources and burial conditions prevailed during Neolithic times. Agreeing with archaeological observations and favoured by rapid burial conditions, very limited alterations have been deduced. AMS data revealed the absence of fluid flow in the ash lenses sampled. Demagnetization revealed a stable single NRM component in ashes, a single or two-component NRM in rubefactions and less stable multicomponent behaviour in clays. In ashes, magnetic minerals are likely secondary in origin formed by low-temperature oxidization soon after burning. Although this thermochemical nature of the NRM invalidates the use of these sediments for palaeointensity studies, archaeomagnetic (directional) data can be successfully obtained because the burning and oxidation are closely confined in time.

SUMMARY The reliability of primary natural remanent magnetization (NRM) signals in greigite-bearing sediments has been frequently questioned. Here, we show that the stable NRM in the deep marine Middle to Late Miocene sediments at Monte dei Corvi, northern Italy, is mainly carried by greigite. Combined rock magnetic experiments and scanning electron microscopy successfully enabled discrimination between two greigite populations. One fine-grained and relatively well-dispersed greigite population (grain size between 60 and 200 nm) is most likely of magnetotactic origin. The second greigite population with larger grain sizes (typically 700 nm to 1 μm) is most likely of authigenic (bacterially mediated) origin, and is related to post-depositional sulphidization processes. Greigite is the main magnetic remanence carrier in the older part of the section (12.8 to 8.7 Ma), whereas greigite and fine-grained (presumably magnetotactic) magnetite are present in the younger part of the section (8.7 to 6.9 Ma). Similar remanent magnetization directions of the magnetite and greigite components, and the likelihood of a magnetotactic origin, suggests that the NRM is of syn-depositional age. We suggest that moderate methane seepage from the underlying sediments may have occurred that resulted in the preservation of pristine greigite. This corroborates the reliability of the previously established magnetostratigraphy at Monte dei Corvi.

article i nfo In this study we interpret the paleo-stress pattern in the Organya Basin (southern Pyrenees, northern Spain) as inferred from the anisotropy of magnetic susceptibility (AMS) of 39 sites distributed over the entire basin. Combined with information from other Cretaceous Iberian basins, such analysis adds to constrain kinematic reconstructions of the Cretaceous rotation of Iberia allied with the opening of the Bay of Biscay and the northward propagation of the North Atlantic. The Organya Basin is an inverted Cretaceous basin in the hanging wall of the Boixols thrust. The lithologies are mainly weakly deformed pelagic and hemi-pelagic limestones and marls which recorded the Aptian 35° counterclockwise rotation of Iberia. Three types of AMS fabrics could be distinguished, all representing typical intermediate and tectonic fabrics. EW magnetic lineations dominate in the eastern part of the basin and are related to crustal shortening during the Pyrenean orogeny. This interpretation is consistent with structural cross-sections across the basin showing more intense shortening in the east. In the central part of the basin, approximately NS oriented magnetic lineations are observed, interpreted as the original extensional direction during basin foundering. So, in line with results from previous studies, AMS can still unveil the original extensional direction in an inverted sedimentary basin, something which may be difficult to reconstruct from geological data alone. The original extension direction in the Organya Basin is perpendicular to the Boixols thrust bounding the basin to the south. Correction for the Aptian rotation of Iberia leads us to infer a NE-SW oriented extension direction at the onset of the rotation of Iberia. This extension direction is inconsistent with current plate kinematic reconstructions of the Cretaceous rotation and motion of Iberia. We therefore suggest that, during the opening of the Bay of Biscay and related Iberian rotation, Iberia was in a much more westerly position than assumed in current models, and that the rotation of Iberia was followed by dominantly eastward translation with respect to southern France, prior to N-S convergence and shortening in the Pyrenees since the latest Cretaceous.

SUMMARY To identify remagnetization is essential for palaeomagnetic studies and their geodynamic implications. The traditional approach is often based on directional analysis of palaeomagnetic data and field tests, which may be inconclusive if the apparent polar wander path (APWP) is poorly constrained or if the remagnetization predates folding. In several cases, rock magnetic work, particularly, the measurement of hysteresis loops allows identification of the so-called ‘remagnetized’ and ‘non-remagnetized’ trends. However, for weakly magnetic samples, this approach can be equivocal. Here, to improve the diagnosis of remagnetization, we investigated 192 isothermal remanent magnetization (IRM) acquisition curves (up to 700 mT) of remagnetized and non-remagnetized limestones from the Organy` a Basin, northern Spain. Also, 96 IRM acquisition curves from non-remagnetized marls were studied as a cross-check for the non-remagnetized limestones. A non-parametric end-member modelling approach is used to analyse the IRM acquisition curve data sets. First, remagnetized and non-remagnetized groups were treated separately. Two or three end-members were found to adequately describe the data variability: one end-member represents the high-coercivity contribution, whereas the low-coercivity part can be described by either one end-member or two reasonably similar endmembers. In the remagnetized limestones, the low-coercivity end-members tend to saturate at higher field values than in the non-remagnetized limestones. When the entire data set was processed together, a three-end-member model was judged optimal. This model consists of a high-coercivity end-member, a low-coercivity end-member that saturates at ∼300–400 mT and a low-coercivity end-member that approximately saturates at 700 mT. Higher contributions of the latter end-member appear to occur dominantly in the remagnetized limestones, whereas the reverse is true for the non-remagnetized limestones, so they plot in clearly distinguishable areas. Meanwhile, the IRM curves from non-remagnetized marls show a behaviour similar to that of the non-remagnetized end-member in the limestones. Therefore, this new approach can be a very useful tool to diagnose remagnetization in weakly magnetic limestones and marls. We recommend applying it to other areas of potentially remagnetized low-intensity sediments.

article i nfo Northern Iberia exposes a series of Mesozoic sedimentary basins, whose final formation history is tied to a rotation episode of Iberia during the Aptian, related to the opening of the Bay of Biscay. Many of these basins experienced widespread remagnetization, previously loosely tied to a 'Mid-Cretaceous thermal event'. Here we make use of the improved apparent polar wander path (APWP) of Iberia to narrowly constrain the age of their remagnetization to show that several basins experienced a regional diachronous remagnetization. We apply the small circle intersection (SCI) method to reconstruct the paleomagnetic field direction during remagnetization in the Organya Basin in the Southern Pyrenees, Spain, where the age of remagnetization is stratigraphically well-dated to around the Barremian-Aptian boundary, and where the paleomagnetic direction during remagnetization has been well-established. The resulting direction of D/I=316.8±5.7°/54.8°±3.3° (where the uncertainty in declination is approximated by α95/cos I and the error in inclination by α95) correspondscloselytoarecently improvedAPWPofIberia,providingsupportfortheapplicabilityof thismethod. ApplicationoftheSCImethodallowsustoshowthattheolderportionoftheremagnetizedbedswasalreadytilted

SUMMARY Here, we report an increased cooling coupled with an intensified aridification for the Xiantai fluvio-lacustrine sequence of Pleistocene age (1.9–0.1 Ma) in the Nihewan Basin (North China), known for the occurrence of several hominin sites. The cooling is related to an up-section decrease in propensity to chemical weathering as inferred from an increase in lowfield susceptibility after cycling to 700 ◦ C. Close to 700 ◦ C, reacting chlorite is providing the iron source for newly formed very fine-grained ferrimagnetic minerals which enhances the susceptibility signal. The reactivity of chlorite after annealing at temperatures above 600 ◦ C is documented with X-ray diffraction and the up-section decrease in propensity to chemical weathering is confirmed by the up-section declining illite/chlorite ratios and increasing chlorite and illite contents based on X-ray diffraction analysis on selected samples.

eve basin (southern France), also deposited at low latitudes. Additionally, we critically review published data from low latitude Permian red beds from the same basins. We focus on records with a sufficient number of samples, and acquire, whenever possible, the original data, or—as a second choice—parametrically sampled published site means. We test the distributions of these directions using a statistical secular variation field model (TK03.GAD). The angular standard deviation (ASD) of the virtual geomagnetic pole (VGP) distribution is a measure of (palaeo)secular variation, and we compare our values of ASD from the Permian red beds to those from lavas erupted in the last 5 Myr. Contrary to our previous studies, we now conclude that secular variation during the Permo-Carboniferous Reversed Superchron is slightly, but significantly, reduced at low latitudes compared with the recent field, as based on a latest compilation that supercedes earlier compilations. It also agrees with a new study, which found that PSV, at low palaeolatitudes during the Cretaceous Normal Superchron, was also significantly lower than in periods with frequent reversals.

SUMMARY The San Quintin volcanic field (SQVF) in Baja California (Mexico) consists of at least 13 volcanic complexes with about 30 lava flows. 40Ar/39Ar step-heating and 3He surface exposure dating experiments suggest eruption ages between 22 and 165 ka. Paleodirections of 14 flows reported here are consistent with normal secular variation. Because of low Curie temperatures and thermal alteration starting at ∼350 °C, classical Thellier–Thellier paleointensity (PI) experiments were deemed unsuccessful. Microwave PI experiments were carried out on 139 samples with 97 being interpreted to be successful, yielding values smaller than 20 μT for all sites and averaging 16.5 μT. Another sample set was studied with the multispecimen parallel differential pTRM method; temperatures for acquisition of pTRM were chosen between 200—and 225 °C. In two cases of higher unblocking temperatures, it was set at 480 °C. The experiments were successful for 104 out of 120 specimens with PI values ranging between 14 and 21 μT and clustering around 18 μT. Three noticeably higher values between 28 and 35.7 μT were obtained. Both methods yield in general well comparable PI data with the microwave PI showing a tendency to be slightly smaller than those from the multispecimen PI. The reason for it is presently not known with certainty. Possible causes include 1) incipient thermal alteration, anticipated to be slightly more prominent in the multispecimen method. 2) Cooling time effects: the microwave method involves heating during only 10–30 s compared to 2400 or more seconds in the multispecimen method. This may account for a relative underestimate of the field intensity by about 4 per cent. 3) A general PI underestimate (by ∼4 per cent) inherent to the microwave perpendicular field method applied to multidomain particles in comparison to parallel field methods. 4) A PI overestimate for small multidomain particles in the multispecimen method. However, both PI methods produced very similar results even for difficult materials as the SQVF lavas giving credit to the veracity of the obtained PI values. The multispecimen parallel differential pTRM paleointensity method thus presents a valuable alternative for traditional thermal PI determinations, as it can be applied using standard laboratory equipment. The microwave paleointensity method may yield better results but requires specialized equipment not yet available commercially.

Widespread Cretaceous remagnetization is documented in several Mesozoic basins in North Central Spain. Organya Basin (South Central Pyrenean foreland) is atypical in the sense that the lower part of the rock sequence (Berriasian-Barremian limestones) is remagnetized while the upper portion (Aptian-Albian marls) is not (Dinares-Turell and Garcia-Senz, 2000). Here, this view is confirmed by the analysis of 41 new paleomagnetic sites over the entire basin, so that a 3D view is obtained. Thermoviscous resetting of the natural remanent magnetization can be ruled out, hence the remagnetization is chemical in origin. A positive breccia-test on remagnetized strata constrains the remagnetization age to older than the Paleocene-Eocene, when the backthrust system was active. The remagnetization is argued to have occurred early in the geological history of the Organya Basin either in the elevated geothermal gradient regime during the syn-rift extension or at the earliest phase of the later compression. Burial is considered the most important cause combined with the lithological effect that limestones are more prone to express remagnetization than marls. The observed pressure solution in the remagnetized limestone is likely associated with the remagnetization, whereas it is unlikely that externally derived fluids have played an important role.

SUMMARY Here we report on the sources of magnetic anisotropy in pyrrhotite, an iron sulphide present in many rocks as an important carrier of the Natural Remanent Magnetization. While the magnetic hysteresis parameters of pyrrhotite are well known, the existing database concerning its anisotropy behaviour is patchy and ambiguous. Therefore, a collection of 11 seemingly single crystals of natural pyrrhotite was scrutinized. Before embarking on the anisotropy determinations the set of single crystals was extensively characterized rock magnetically by measuring Curie temperatures, hysteresis loops, IRM acquisition curves, and FORC diagrams (the latter three all at room temperature). First the variation of the low-field susceptibility as function of applied field and grain size was evaluated for fields ranging from 1 to 450 A m −1 . Existing grain size dependent data and the present larger crystals show a logarithmic grain size dependence. This enables estimating the grain size for unimodal pyrrhotite distributions in rocks. Measured trends are better fitted with an exponential function than with a Rayleigh Law style function. Based on the rock magnetic characterization and the behaviour of the anisotropy of magnetic susceptibility six samples (of the original 11) were selected for the high-field anisotropy determinations within the basal plane. Those data were acquired with a torque cantilever-type magnetometer. As expected, most single crystals showed a pure 6‐ θ curve within their basal plane because of the easy axis configuration. In some crystals, however, lower harmonic terms overlapped the 6‐θ term. This may be the dominant source of the observed variation in magnetic anisotropy properties. Torque data of three of the six samples were of sufficient quality to allow evaluation of K 1. Re-evaluation of existing torque data and including the present newly derived determinations, yields for the anisotropy constant of pyrrhotite within the basal plane K 1: (2.7 ± 0.2) 10 4 Jm −3 . This is over an order of magnitude more precise than the sparse existing K 1 data; only the value reported by Mikami and co-authors in 1959 agrees with the new determination. With this firmly established K 1 value meaningful anisotropy models are now possible for pyrrhotite-bearing rocks.

SUMMARY During the Miocene‐Pliocene, the Carpathian region represented the westernmost part of the so-called Eastern Paratethys, a palaeobioprovince that covered central and eastern Europe as well as parts of southwest Asia. Previous palaeomagnetic investigations provide a highresolution magnetochronology for the sedimentary sequences of the Carpathian foredeep and indicate a marked transition in magnetic carriers from iron oxides to iron sulphides, in Chron C3r. Here, we demonstrate using detailed rock magnetic investigations and scanning electron microscope (SEM) analyses that the major magnetic iron sulphide mineral is greigite (Fe3S4). Thermomagnetic runs indicate an irreversible decrease in magnetization with increasing temperature up to 400 ◦ C and SEM observations indicate octahedral grain morphologies and Fe:S ratios that are indicative of greigite. Hysteresis loops have ‘rectangular’ shapes, which are typical of single domain behaviour, with coercivity and coercivity of remanence at values of Bc = 35‐45 mT and Bcr = 52‐67 mT, respectively. First-order reversal curve diagrams have contours that close around single domain peaks with Bc values of 45‐90 mT. Isothermal remanent magnetization component analysis reveals a small dispersion for the greigite component (dispersion parameter, DP ∼ 0.15 log units) indicating a narrow grain size distribution. A positive fold test, inclination shallowing and two positive reversal tests are additional arguments for the syn-depositional formation of this greigite. We thus argue that (most of) the greigite was formed under early diagenetic conditions, that is, within 1000 yr of deposition of the sediment in this setting, and that it thus can be considered as a reliable recorder of the palaeomagnetic signal in this situation. The appearance of greigite during Chron C3r (between 6.0 and 5.5 Ma) in the Carpathian foredeep is most likely related to regional tectonic and/or climatic events that reshaped the basin configuration and changed the palaeoenvironmental conditions.

SUMMARY Low-temperature rock magnetic measurements have distinct diagnostic value. However, in most bulk marine sediments the concentration of ferrimagnetic and antiferromagnetic minerals is extremely low, so even sensitive instrumentation often responds to the paramagnetic contribution of the silicate matrix in the residual field of the magnetometer. Analysis of magnetic extracts is usually performed to solve the problems raised by low magnetic concentrations. Additionally magnetic extracts can be used for several other analyses, for example electron microscopy or X-ray diffraction. The magnetic extraction technique is generally sufficient for sediments dominated by magnetite. In this study however, we show that high-coercivity components are rather underrepresented in magnetic extracts of sediments with a more complex magnetic mineralogy. We test heavy liquid separation, using hydrophilic sodium polytungstenate solution Na6[H2W12O40], to demonstrate the efficiencies of both concentration techniques. Low-temperature cycling of zero-field-cooled, field-cooled and saturation isothermal remanent magnetization acquired at room temperature was performed on dry bulk sediments, magnetic extracts, and heavy liquid separates of clay-rich pelagic sediments originating from the Equatorial Atlantic. The results of the thermomagnetic measurements clarify that magnetic extraction favours components with high spontaneous magnetization, such as magnetite and titanomagnetite. The heavy liquid separation is unbiased with respect to high- and low-coercive minerals, thus it represents the entire magnetic assemblage.

In paleomagnetic and environmental magnetic studies the magnetomineralogical identification is usually based on a set of rock magnetic parameters, complemented by crystallographic and chemical information retrieved from X-ray diffraction (XRD), (electron) microscopy or energy dispersive spectroscopy (EDS) of selected samples. While very useful, each of these supplementary techniques has its limitations when applied to natural sample material which are related to low particle concentrations (down to the ppm range in marine sediments) and very fine grain sizes (down to the nm scale). Therefore, meaningful application of such techniques depends on sample quality. Electron backscatter diffraction (EBSD) of individual grains in scanning electron microscopy (SEM) enables mineralogical identification of grains down to ∼0.2 micrometer and is particularly powerful when combined with EDS. In this study, we show the merits of EBSD for rock magnetic investigations by analyzing titanomagnetites and hemoilmenites of various compositions and submicron lamella of titanomagnetite-hemoilmenite intergrowths. Such particles often occur in natural marine sediments where EDS often has a semi-quantitative character and compositionally similar intergrowths may be difficult to distinguish. With the mineralogical information provided by EBSD unambiguous identification of spinel-type and trigonal oxides is obtained. Optimal EBSD patterns are gathered from smooth, polished surfaces, but here we show that interpretable EBSD patterns can be obtained directly from the surface of unconsolidated, so called 'non-embedded' particles from marine sediments. This information enhances the interpretative value of rock magnetic parameters. © 2007 The Authors Journal compilation © 2007 RAS.

Knowledge of palaeointensity variation is required for determining the full vector variation of the geomagnetic field as a function of geological time. This provides essential constraints for numerical geodynamo models. To date, most palaeointensity determination methods are laborious, characterised by rather low success rates, and demand substantial processing time. The rocks under investigation must obey stringent criteria to yield faithful palaeointensities: the magnetic particles must be single domain, the natural remanent magnetisation must be a thermoremanent magnetisation, and during the successive heating steps in the laboratory no chemical alteration should occur. Here, we describe a new method that allows all magnetic domain states to be processed, i.e. it does not require single domain particles. The method makes use of the linearity of partial thermoremanent magnetisation (pTRM) with the applied laboratory field. Multiple specimens are used so that every sample is exposed only once to a laboratory field, warranting that all samples experienced the same magnetic history. Through the limited number of thermal steps alteration effects are reduced as well. The laboratory pTRM and natural remanent magnetisation (NRM) of the specimens are oriented parallel to minimise the effects of high-temperature tails that affect multidomain minerals. The pTRM acquisition temperature is selected below the temperature at which chemical alteration sets in and above the temperature trajectory where secondary viscous NRM components occur. The procedure requires a lower number of steps than any other palaeointensity method, reducing significantly the total time needed per rock unit. We propose to name the new protocol ‘multispecimen parallel differential pTRM method’ .I t provides the correct answer to ∼5% for artificial samples and natural rocks containing multidomain magnetic particles that were given a laboratory TRM of known intensity. Application to the Paricutin September–December 1943 lava flow (three sites, 7 specimens per site) yields a weighted mean of 45.9±1.25 μT, within uncertainty margin of the expected value of 45.0 μT.

Pyrrhotite bearing metamorphic limestones have recently experienced an increasing relevance in paleomagnetic research. Simple univectorial remanences document the metamorphic uplift, whereas more complex multicomponent pTRMs may constrain its age. For a successful application of the latter, it is important to estimate the degree of magnetic interactions to ensure the additivity of individual pTRM segments. We therefore have subjected the sized dispersed suite (

SUMMARY We report on the magnetic properties and the acquisition of a chemical remanent magnetization (CRM) in a field of 100 μT as a function of temperature and time during the lepidocrocite–maghemite–haematite reaction chain. The development of CRM was monitored at a series of 13 temperatures ranging from 175 to 550 °C; data acquisition was done at the specific formation temperatures for durations of up to 500 hr. Up to acquisition temperatures of 200 °C it takes a considerable time (up to 7 hr) before the CRM is measurable. This time decreases with increasing temperature, reflecting the activation energy of the reaction to form the first maghemite. During the lepidocrocite conversion, formation of two types of maghemite is suggested by two peaks in the CRM versus time curves. Magnetic properties were analysed after various stages in the reaction. They indicate a mixture of superparamagnetic and single-domain maghemite. The first reaction product (obtained after annealing at 200 °C) is a fine-grained yet crystalline maghemite (labelled type A). Before massive maghemite formation occurs, the coercive and remanent coercive forces go through a minimum at intermediate temperatures of 250–300 °C (annealing for 2.5 hr). This minimum lowers to 200–250 °C with increasing annealing time (500 hr). This is probably the result of two processes acting simultaneously—formation of superparamagnetic maghemite particles of a second less crystalline maghemite type (labelled type B) and removal of stacking faults in type A maghemite. The second process is suggested by analogy to the behaviour of natural magnetite/maghemite systems on annealing. Removal of stacking faults is reported to result in a magnetic softening of the grain assemblage. Annealing at 300–350 °C removes most of the lepidocrocite and the second maghemite type, type B, becomes prominent. Haematite formation sets in at slightly higher temperatures, yet the type B maghemite is in part thermally stable up to 600 °C enabling Thellier–Thellier experiments. This stability is also inferred from Arrhenius fitting that shows a comparatively high activation energy for the maghemite to haematite reaction. In Thellier–Thellier experiments the CRM showed a markedly downward convex Arai–Nagata plot while a second thermoremanent magnetization (TRM) showed perfect linear behaviour as expected. This feature may be used to recognize CRM in natural rocks.

To further evaluate the potential of magnetic anisotropy techniques for determining the origin of the natural remanent magnetization (NRM) in sedimentary rocks, several new remanence anisotropy measurement techniques were explored. An accurate separation of the remanence anisotropy of magnetite and hematite in the same sedimentary rock sample was the goal. In one technique, Tertiary red and grey sedimentary rock samples from the Orera section (Spain) were exposed to 13 T fields in 9 different orientations. In each orientation, alternating field (af) demagnetization was used to separate the magnetite and hematite contributions of the high field isothermal remanent magnetization (IRM). Tensor subtraction was used to calculate the magnetite and hematite anisotropy tensors. Geologically interpretable fabrics did not result, probably because of the presence of goethite which contributes to the IRM. In the second technique, also applied to samples from Orera, an anisotropy of anhysteretic remanence (AAR) was applied in af fields up to 240 mT to directly measure the fabric of the magnetite in the sample. IRMs applied in 2 T fields followed by 240 mT af demagnetization, and thermal demagnetization at 90°C to remove the goethite contribution, were used to independently measure the hematite fabric in the same samples. This approach gave geologically interpretable results with minimum principal axes perpendicular to bedding, suggesting that the hematite and magnetite grains in the Orera samples both carry a depositional remanent magnetization (DRM). In a third experiment, IRMs applied in 13 T fields were used to measure the magnetic fabric of samples from the Dome de Barrot area (France). These samples had been demonstrated to have hematite as their only magnetic mineral. The fabrics that resulted were geologically interpretable, showing a strong NW-SE horizontal lineation consistent with AMS fabrics measured in the same samples. These fabrics suggest that the rock’s remanence may have been affected by strain and could have originated as a DRM or a CRM. Our work shows that it is important to account for the presence of goethite when using high field IRMs to measure the remanence anisotropy of hematite-bearing sedimentary rocks. It also shows that very high magnetic fields (>10 T) may be used to measure the magnetic fabric of sedimentary rocks with highly coercive magnetic minerals without complete demagnetization between each position, provided that the field magnetically saturates the rock.

SUMMARY A number of recent studies have applied the cumulative log Gaussian (CLG) modelling tech- nique to bulk isothermal remanent magnetization (IRM) curves to investigate the coercivity contributions of different minerals contained within a natural sample. Here, we present a series of Preisach-Neel models used to investigate how robust the assumption of fitting lognormal coercivity distributions is in the presence of local interaction fields and thermal relaxation. Our models indicate that the starting state of the magnetic system, magnetic interaction and thermal relaxation have a strong influence on the form of IRM curve, meaning that in a sub- stantial number of cases the lognormal assumption fails and the CLG procedure can produce misleading interpretations. In some cases the failure of the CLG assumptions produces models which introduce spurious additional coercivity components into the fitted curve.

Iron is distributed over different minerals (i.e. silicates, pyrite, detrital oxides) that are present in a sediment sequence that formed under anoxic conditions. After post-depositional re-oxidation of the sediments pyrite is no longer present and diagenetic iron phases constitute an important portion of the iron in the oxidised part of the sapropel. They are very fine-grained making them amenable to analysis by means of sequential extraction and mineral-magnetic methods. The sequential extraction shows that besides iron in silicates, iron mainly occurs in ‘amorphous’ oxides in the oxidised part of the S1 sapropel. Pyrite constitutes an important fraction in the still reduced part of the S1 sapropel. Some silicon is dissolved during the extraction for the ‘amorphous’ oxides, suggesting that ‘amorphous’ iron also occurs as ferro-silicate coatings. Mineral-magnetic analysis involved component analysis of the isothermal remanent magnetisation (IRM) and hysteresis loop measurements. Three coercivity phases could be identified in the IRM component analysis; these were interpreted as ‘detrital’ magnetite, hematite, and biogenic magnetite. The diagenetically formed iron phases influence the parameters of the IRM components. Hysteresis measurements together with the IRM component analysis, indicate the importance of bacterial magnetite in the oxidised sapropel, particularly in the lower part of the active oxidation zone.

SUMMARY An earlier study of the cyclic Miocene red bed sequence of La Gloria (Spain) by Krijgsman et al. indicated complicated behaviour of the natural remanent magnetization (NRM) in parts of the section, particularly close to reversal boundaries. We resampled part of the section with high resolution and used extensive rock-magnetic analyses to characterize the magnetic remanence carriers. Below a conspicuous hydromorphous layer, the magnetic contributions of haematite and magnetite are approximately equal, while in the brown layers on top of the hydromorphous layer the contribution of magnetite is drastically reduced. This is probably related to a change in hydrological conditions. The NRM is characterized by: (1) a low-temperature (200‐360 ◦ C) overprint of unknown age, (2) a medium-temperature (360‐580 ◦ C) component, interpreted as the characteristic remanent magnetization (ChRM); and (3) a high-temperature haematite component (600‐ 680 ◦ C), carrying the same directions as the medium-temperature component. There is no perceivable delay in NRM acquisition between the medium- and the high-temperature components; they are both regarded as primary. The behaviour of the NRM seems to be similar in the cyclic lithologies. The acquisition of NRM thus seems to be independent of lithology in the cyclic part of the section. The higher sampling resolution yielded the detection of a new polarity zone, which probably represents a geomagnetic feature according to rock-magnetic properties. However, the more detailed magnetostratigraphy of the resampled part of the section indicates that the earlier correlation to the geomagnetic polarity timescale is no longer tenable. Therefore, the La Gloria section should no longer be used as a magnetostratigraphic dating tool of mammal biochronology.

Siderite (FeCO3), rhodochrosite (MnCO3), and vivianite ((Fe3(PO4)2) � 8H 2O) are well-known authigenic minerals in a number of sedimentary settings. Here, we explore the potential of low-temperature mineral magnetic techniques for their identification at low concentration in bulk samples thus expanding mineral magnetic proxies for environmental purposes. The basic rock magnetic properties of these minerals, which are paramagnetic at ambient temperature, were determined with aMagnetic Properties Mea- surement System'. Well-crystalline chemically analyzed material of natural origin was used to gather these data. The diagnostic value of the observed specific magnetic properties was tested on two mid-Eocene sediment samples from the Norwegian Sea (ODP Leg 104, Site 643) known to contain these minerals. The observed Ntemperatures of siderite (37 K) and rhodochrosite (34 K) conform with literature data. Both carbonates show a fairly strong spin-canted remanence (� 0.4 Am 2 /kg) from cooling in a 5 T magnetic field. Different ratios of field-cooled and zero- field-cooled remanences, however, allow a discrimination between the two minerals. A characteristic of rhodochrosite is its ex- tremely high magnetic susceptibility just below the Ne temperature. An almost vertical slope in very low fields of the hysteresis loop also testifies to this high susceptibility. It is assigned to a weak anisotropic ferromagnetism confined to the basal plane in which the spontaneous magnetization can almost freely rotate. A prominent magnetic property of siderite is its metamagnetism, resulting in a progressively upward bending of the hysteresis curve in magnetic fields above 5 T. Vivianite also shows an onset of meta- magnetic transition below 5 K in 5 T fields and atwo-stage' increase in susceptibility between 2 and 12 K attributed to successive short- and long-range magnetic ordering. The magnetic properties of the two authigenic marine sediments could be largely explained by combining characteristics of rhodochrosite and manganosiderite. Shifting of the Npoints to lower temperatures and less well pronounced magnetic phenomena are attributed to element substitution and non-stoichiometry which occur commonly in sedi- mentary environments. � 2003 Elsevier Ltd. All rights reserved.

A data set of room temperature magnetic parameters for several iron oxides and sulphides was compiled from the available literature. The aim was to propose the most effective methods for assessing mineralogy, concentration and domain state within environmental magnetic studies. Establishing the magnetic mineralogy is essential for a correct interpretation of concentration and grain-size indicative parameters. Hematite and in particular goethite are recognised by their high (remanent) coercivities. The ratio of saturation remanent magnetisation to susceptibility is suited for assessing mineralogy; for low coercivity minerals, high values of the ratio are indicative of pyrrhotite. Greigite and maghemite both have intermediate ratios, while very low values suggest the presence of (titano)magnetite. From the concentration-dependent parameters mass specific susceptibility, saturation remanent magnetisation and susceptibility of anhysteretic remanent magnetisation, susceptibility displayed the least grain-size dependence. Therefore, it is perhaps the best indicator of concentration, under the proviso of uniform mineralogy and when paramagnetic and diamagnetic contributions are taken into consideration. All minerals showed a decrease in coercivity and remanence ratios with increasing grain size for sizes larger than one micrometer. In contrast, the coercivity of goethite showed an increase with grain size. Assessment of domain state was complicated by very small and large grains displaying similar magnetic properties.

The Middle Ordovician Volginsky and Kirensky fossil zones were sampled in the Krivaya Luka section (Krivolutsky suite) that outcrops along the Lena river in Siberia. The Volginsky and Kirensky zones are coeval to the Llandeilo in the global geologic time scale. The Krivaya Luka section consists of siltstones, clays, sandstones, and limestones, and displays a remarkably distinct sedimentary cyclicity, especially in its reddish middle part.

We have investigated variations in the natural remanent magnetisation (NRM) in red beds which were deposited during the Permo-Carboniferous Reversed Superchron (PCRS) near the paleo-equator. These red beds are reliable recorders of the geomagnetic field, because the NRM is carried by detrital hematite. We analysed the angular standard deviation (ASD) and spectral content of the directional records to quantify the amount of paleosecular variation (PSV) during the time of deposition. The ASD of a high-resolution directional record of the Dome de Barrot which spans approximately 27 kyr is 10°. This is comparable to recent (0–5 Ma) low-latitude lavas from the PSVRL database ( McElhinny and McFadden, 1997 ) and to recent lake sediments from the SECVR database ( McElhinny and Lock, 1996 ). The frequency spectra of declination and inclination are also similar to those from the recent lake sediments: spectral power significant at the 95% level is concentrated in periods of ∼1–5 kyr, higher frequencies are not significant. From the ASD and spectral analyses, it appears that the amount of PSV during—at least part of—the PCRS was comparable to that of recent times. Several heuristic and numerical geodynamo models are discussed in the context of Superchrons and PSV. There is a discrepancy between the amount of SV shown by our data and that suggested by numerical models which use lateral heat flux variations at the core–mantle boundary (CMB). However, computational limitations still force some model parameters to have values that are unrealistic for the Earth. For the phenomenological geodynamos, ‘low-energy state’ models which suggest that PSV is suppressed during a Superchron are not supported by the Dome de Barrot data. Rather, a ‘high-energy state’ of the geodynamo, in which outer-core convection is vigorous, is favoured as a prerequisite for a Superchron to occur.

SUMMARY Chemical extraction techniques are being introduced into environmental magnetism studies to aid in the interpretation of magnetic climate proxies. Previous studies have shown that the acid-ammonium-oxalate/ferrous-iron (AAO‐Fe 2+ ) extraction technique can be used to selectively dissolve very fine-grained magnetite and maghemite from synthetic samples. Here, we present the results of a study of this extraction technique to serve as a tool for selective dissolution of pedogenic magnetic minerals from a loess‐palaeosol transect. Before and after extraction, the samples were subjected to classic mineral‐magnetic methods (measurement of low-field susceptibility and hysteresis parameters), as well as first-order reversal-curve analysis. In addition, acquisition curves of the isothermal remnant magnetization (IRM) were fitted with logarithmic distributions. These analyses showed the magnetic dominance of lowcoercivity magnetic minerals. By subtracting the IRM remaining after extraction from that before extraction, the magnetic fraction that was dissolved could be characterized as well, supplying extra information concerning the performance of the extraction technique. The AAO‐Fe 2+ method successfully dissolved the superparamagnetic and part of the singledomain material from the palaeosol samples in one extraction step, repeating the extraction resulted in hardly any further changes to the magnetic content of the samples. The magnetic characteristics of the loess samples remained stable throughout the extraction experiment. The combination of the AAO‐Fe 2+ extraction with mineral‐magnetic analysis has successfully identified the pedogenic contribution in our samples. Therefore, variations in the lithogenic fraction potentially present in pedogenically enhanced intervals can be assessed, improving the merit of mineral‐magnetic climate proxy parameters.

We present a Monte Carlo based method for the determination of errors associated with frequency spectra produced by the CLEAN transformation of Roberts et al. (1987) . The Monte Carlo procedure utilises three different types of simulation involving a data stripping operation and the addition of white and red noise to the analysed time series. The simulations are tested on both synthetic and real data sets demonstrating the ability of the procedures to extract coherent information from time series characterised by the low signal-to-noise-ratio that is typical of many palaeoclimatic records. Significance levels derived for the Monte Carlo spectra of four time series from the Vostok ice core are utilised in the study of eccentricity components contained within the palaeoclimatic archive since ∼420 ka. Inversion of the Vostok frequency spectra into the time domain reveals the differing influence of orbital parameters in the palaeoclimatic proxy records as well as the relative magnitudes of the eccentricity components contained in the time series of greenhouse gas concentration, ice volume and local temperature.

SUMMARY The acquisition of isothermal remanent magnetization (IRM) curves through the application of stepwise-increasing uniaxial fields to a rock-magnetic sample provides an important non-destructive tool for the investigation of coercivity spectra (Dunlop

Mineralogical discrimination of iron oxides in soils and sediments is not a trivial task, mainly because of their small grain size and low concentration. With mineral–magnetic techniques, highly magnetic ferrimagnetic spinels can be determined with a very low detection limit (∼10 ppm). Unfortunately, the magnetic signal of natural samples is often dominated by magnetite, and in particular the expression of weakly magnetic antiferromagnetic minerals is suppressed by this high signal. In contrast, electrochemical techniques, such as voltammetry of microparticles (VMP) are not affected by these differences in magnetic signal. VMP makes use of the electrochemical law that iron(oxy)(hydr)oxides can be reductively dissolved at potentials that are specific for their mineralogy and reactivity. Therefore, by scanning over a potential range and monitoring the potential at which a reaction occurs, one can specify the type of minerals present. VMP can be applied to samples without requiring any form of pre-treatment. The method is rapid, comparatively straightforward to interpret, and requires little amount of sample (μg). However, the detection limit of VMP on natural samples has not yet been investigated. Here, we present a pilot study of the use of VMP in the identification of the ferric oxides of natural samples from Spanish red beds of Miocene age. A standard addition method with pyrolusite as internal standard enabled semi-quantitative analysis of the samples; concentrations of (anti)ferromagnetic iron(oxy)(hydr)oxides down to ∼0.1 wt% could be detected.